Leon Steenkamp

Building small satellites on the tip of Africa. My other ride is a CubeSat.

Open water swim tracker - tracker hardware

This post is part of the series detailing the Open water swim tracker project - more info here.

The tracker hardware is placed on the boat that accompanies the swimmer and periodically sends GPS data over its LTE link to the project server.

The tracker was mostly built from parts that were on hand already. The items that needed to be procured were the USB power bank, waterproof enclosure, and LTE modem. These items were sourced from local online retailers.


The tracker hardware inside enclosure

Tracker components:

OLED screen for monitoring (SSD1306)
USB power bank for power
u-blox GPS receiver (LEA-5H-0-005)
Alcatel USB LTE modem, Vodacom SIM card
Raspberry Pi 4 - 2GB

Initially, the Raspberry Pi (RPi) was not too happy with the power supply, with occasional reboots being noticed. After shortening the USB cable used for testing, the reboots stopped. The cable also needed to be shortened to fit inside the final enclosure. The power supply was always going to be marginal because the GPS receiver and LTE modem were also being powered from the RPi’s USB, but after testing, the configuration was stable enough to continue.

It was quite tricky to find a USB dongle type LTE modem that I felt confident would work under Linux and was a reasonable price. These USB LTE modems, it seems, are not that readily available anymore. WiFi-based LTE modems with a built-in battery seem to be the more popular option available online. It is possible to retrieve various statistics from the webpage that is hosted by the LTE modem. This date is continuously logged and covered further in another post later.

The u-blox GPS evaluation board is one I have had for a few years and was bought from RF Design. The GPS receiver performs reasonably well, but newer models would probably perform better and support more GNSS (Global navigation satellite system) constellations - the current receiver only supports GPS.

The addition of the OLED screen worked very well because it allows the user to check the device status easily in the field. Information relating to the GPS, LTE connection, and Internet connection can be checked without having to log into the device remotely. The screen cycles through several pages with status information. Very handy addition.


OLED screen close up - no GPS lock

The addition of a power switch (after initial testing) was a big quality-of-life improvement and quite obvious in retrospect. I have my doubts about the switch actually being waterproof, but that is something to look at in the future (a big sandwich bag keeps things extra dry in the meantime).

For a first revision, the tracker hardware worked very well and survived a number of multi-hour tests — one being across False Bay! The current tracker configuration runs for just over eight hours in real-world conditions. Testing of the hardware is covered in a later post.

Open water swim tracker - tracker hardware

Other posts in the series